1. Field of the Invention
The present invention relates to a semiconductor storage device.
2. Description of the Related Art
As the recent increase in capacity of SRAMs, the number of memory cells connected to one bit line has also increased, providing greater impact on the SRAMs due to bit-line capacitance. A large bit-line capacitance could cause adverse effects, such as a delay in changing potentials of the bit lines in read operation or corruption of retained data in memory cells due to disturbance, etc. If the bit lines are divided into short sections to prevent such adverse effects, the area occupied by sense amplifier circuits becomes larger in the SRAM, which would present difficulties in achieving higher capacity.
To this extent, a so-called “single-bit-line reading architecture” is known to detect the potential of only one of a pair of bit lines while dividing bit lines into short sections, instead of providing a sense amplifier circuit of differential amplifier type for differentially amplifying the potentials of a pair of bit lines, as disclosed in, e.g., “The Asynchronous 24 MB On-Chip Level-3 Cache for a Dual-Core Itanium®-Family Processor” (2005 ISSCC). In this publication, the single-bit-line reading architecture is employed in the SRAM, wherein a read circuit and a write circuit are arranged in the same area in the center of cell arrays and a plurality of columns are connected to a single read circuit and write circuit.
Column switches that connect the respective read and write circuits to the corresponding columns have very large impact on the reading speed. Therefore, in accelerating reading operations, a read circuit and a write circuit are required for each column in order to omit the column switches. In this case, however, it becomes more difficult to achieve reduction in area due to the increased wiring congestion. In addition, the bit lines also have higher wiring density and become longer than required, which would result in a larger bit-line capacitance and degradation in performance of the SRAM.